The freeze point of a fuel composition is an important factor in determining whether it is suitable for use in power units which are intended for operation under low temperature conditions, such as for example arctic conditions. It is also an important factor in relation to aviation use, for which low temperature conditions are experienced at high altitudes. It is clearly vital that the fuel composition does not freeze or cause flow to be restricted (because of increased viscosity or blocked filters) during operation, otherwise the consequences could be disastrous.
Additives are known for inclusion in fuel compositions to enable them to be used under such low temperature conditions. Such additives include flow improver additives and wax anti-settling agents. However, it would be desirable to be able to achieve the low temperature effects of such additives whilst reducing, or even eliminating, their presence.
In “Qualification of Sasol semi-synthetic Jet A-1 as commercial jet fuel”, SwRI-8531, Moses et al., November 1997, is described the blending into jet A-1 fuel of a synthetic iso-paraffinic kerosene (IPK), derived from synthesis gas through a Fischer-Tropsch process. IPK is described as having a very low freezing point, which is stated to be typically less than −60° C. Blends of 25% and 50% IPK in Jet A-1 are described as having freeze points of above −60° C., but below the freezing point of Jet A-1, which is indicated to be −47 to −49° C. Therefore, the freeze points of the blends lie between the respective freeze points of the blend components. This document also refers to the freeze points of blends of SMDS (i.e. Shell Middle Distillate Synthesis) kerosene with conventional fuels always being lower than predicted by blending ratio, i.e. below that according to a linear blending formula, but with no reference to where the freeze points of the blends lie in relation to the freeze points of the blend components. Therefore, from the disclosure of this document it would not be expected that the freeze point of blends would lie below the freeze points of both of the blend components.
In “Freezing point of jet fuel blends”, Schmidt, Minutes of the meeting of the low temperature flow performance of aviation turbine fuels group, CRC Aviation fuel, lubricant and equipment research meeting, April 1995, there is discussion of the relationship of the measured freeze points of various jet fuel blends in relation to linear blending assumptions. It is shown in this document that said freeze points could be higher than or lower than the freeze points based on linear blending assumptions, and can be between the freeze points of the blending components or below the freeze points of both of the blend components. Thus, it is not possible to predict from this document what the relationship will be between the freeze point of a blend and the freeze points of the blend components, particularly of blends in which one of the components is a Fischer-Tropsch derived fuel, such fuels not being mentioned in this document.